What are you talking about? Panini did detail the pronunciation of each character. And the actual pronunciation (i.e. accounting for things like assimilation) i

What are you talking about? Panini did detail the pronunciation of each character. And the actual pronunciation (i.e. accounting for things like assimilation) is then encoded in the writing via Sandhi. Likewise, we have older information on pronunciation as well in the Pratishakhyas. Ancient Indians cared a lot about accurate pronunciation, especially for mantras where the thinking was that without correct pronunciation, the mantra would not work. Picture the Harry Potter "wingardium leviosa" scene. Modern don’t actually hew to the prescribed pronunciation for certain letters, but it’s certainly specified by Panini.